Electroluminescent lamp having envelope of water-impermeable plastic having hydrophilic plastic liner



Sept. 8, 1964 J. c. DEVOL ETAL 3,148,299

ELECTROLUMINESCENT LAMP HAVING ENVELOPE OF WATER-IMPERMEABLE PLASTIC HAVING HYDROPHILIC PLASTIC LINER Filed Jan. 4, 1961 PEICENT 0F lN/f/AA BB/GHTNESS 4 ms Homes INVENTORS j E J5me: C. Deu-a/ law/"6170a 5 Greer/e United States Patent Ofi ice 3,148,299 Patented Sept. 8, 1964 3,143,299 ELECTRQLUMWESCENT LAMP HAVENG ENVE- LQPE 8F WATER-EVLPERMEABLE iLASTiC HAVER G HYDRGPHELEC PLASTEC LHNER James C. Devol and Lawrence E. Greene, Zieveiand Heights, Shin, assignors to General Electric Company, a cerperaficn of N ew York Fit-er Jan. 4, 1961, Ser. No. 89,613 Claims. (ill. 313T.63)

This invention relates to electroluminescent cells or lamps, that is to devices in which an electroluminescent phosphor is excited by an electric field to emit light, and is particularly concerned with a construction for improv* ing the resistance to water depreciation.

An electroluminescent cell generally comprises a layer of phosphor capable of generating light under the action of an electric field, sandwiched between a pair of conducting layers at least one of which is transparent or at least light-transmitting. One form of electroluminescent lamp now well-known is described and claimed in Patent 2,945,976, Fridrich and Dell, Electroluminescent Lamp and Manufacture Thereof, assigned to the same assignee as the present invention. That lamp comprises a flexible laminated assembly of electrically active layers encased in a thin envelope of thermoplastic material evacuated and heat-sealed around the edges. The electrically active elements comprise an aluminum foil coated with a layer of high dielectric constant material which in turn is overcoated with a layer of electroluminescent phosphor and finally overlaid with a transparent conducting sheet such as a sheet of conducting fibers. The aluminum foil and the conducting paper form the electrodes of the lamp to which an alternating potential is applied and light is emitted through the glass paper and plastic envelope.

The known electroluminescent phosphors are extremely sensitive to water vapor and depreciate and darken rapidly in its presence. For this reason, practical forms of electroluminescent lamps include some means for sealing or encapsulating the phosphor in order to exclude moisture. However even the most impermeable of the known plastic sheet or film materials are not sufiiciently impermeable to exclude water vapor completely over the life of an electroluminescent lamp. It has been observed for instance that even where the thermoplastic encapsulating envelope of a laminated electroluminescent lamp consists of high density polyethylene, or of yet more water imermeable polychlorotrifiuoroethylene (KEL-F), deterioration of the lamps gradually sets in after several hundred hours of operation in a maximum humidity environment. Of course the advent of such deterioration may be further delayed by resorting to the use of thicker sheets for the encapsulating envelope but this is not economically attractive.

The object of the invention is to provide a construction for improving the resistance to water depreciation, and therefore the life and maintenance, of electroluminescent lamps encapsulated in materials which are not totally impervious to water vapor.

A more specific object of the invention is to provide an arrangement for improving the resistance to water depreciation of plastic encapsulated electroluminescent lamps wherein the electrically active layers are encased in a thin envelope of thermoplastic material which is evacuated and heat-sealed around the edges.

In accordance with the invention, I have discovered that for the purpose of preventing the deleterious effect of moisture penetration into an electroluminescent lamp, it is much more eifective to use an outer sheet of highly impermeable plastic material and to back it up with an inner sheet or lining of another plastic material having an affinity for water or a disposition to retain water, that is, a hydrophilic sheet, than merely to increase the thick ness of the highly impermeable sheet. The hydrophilic sheet additionally should have the property of cementing or adhering to the outer sheet and should fill up the pores in the fibrous conducting sheet when the components are laminated together. For example, polychlorotrifluoroethylene is one of the most Water-impermeable plastic materials and is suitable for the outer encapsulating sheet. Rather than increasing the thickness of the outer encapsulating sheet, I have found it preferable to back it up with a hydrophilic sheet or layer, and the combination of sheets is much more effective than an equivalent thickness of the relatively highly impermeable polychlorotrifiuoroethylene used alone.

The preferred material for the hydrophilic sheet is a polymer of caprolactam, that is, the polyamide commonly known as nylon 6. With this particular material, another surprising efiect or benefit which has been observed, over and above the improved resistance to water depreciation, is an increase in brightness where the nylon sheet is placed next to a layer of conductive glass fibers forming the transparent electrically conductive electrode of the lamp.

For further objects and advantages and for a detailed description of a preferred lamp construction embodying the invention, attention is now directed to the following description and accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a pictorial View of a flexible laminated electroluminescent lamp embodying the invention.

FIG. 2 is a graph illustrating the improvements in resistance to Water depreciation realized by the invention.

Referring to FIG. 1, there is shown an electroluminescent lamp 1 having one corner delaminated or peeled open to show the constituent layers and the internal construction. The thickness of the layers has been exaggerated for the sake of illustration. The lamp or cell is flat and rectangular and is made up of flexible components laminated together and entirely sealed in plastic material of relatively high impermeability. The lamp is energized by applying an alternating voltage, for instance 118 voltscycle A.C., to the copper screen terminals 2, 3 projecting laterally from the edge of the plastic envelope. The underside and the topside of the lamp envelope, consisting of sheets 4, 5 of plastic material which flows under heat and pressure, are welded or heat-sealed together along the margins. Suitable thermoplastic materials of relatively high impermeability to moisture consists of high density polyethylene film, or polychlorotrifluoroethylene film. Polytetrafiuoroethylene may be used with a suitable cement. Polychlorotrifluoroethylene is preferred for optimum stability and impermeability to moisture; it is manufactured by Minnesota Mining and Manufacturing Company and sold under the designation KEL-F; a suitable film thickness is .005 inch. Inasmuch as the underside sheet 4 is backed up by a metal foil 6 which is itself water impermeable, it need not have the same high degree of water impermeability as the topside sheet 5.

The electrically active elements comprise a rectangular sheet of metal foil 6, for instance full-soft annea ed aluminum of .0025 inch thickness, coated with an insulating layer 7 (indicated by cross-lining) of high dielectric constant material, the latter overcoated with a lightproducing layer 8 (indicated by speckling) of electroluminescent phosphor. Foil 6 is placed over the lowermost plastic sheet 4 leaving a clear margin all around. Insulating layer '7 may consist of barium titanate dispersed in an organic polymeric matrix of high dielectric constant such as cyanoethyl cellulose plasticized with cyanoethyl phthalate according to Patent 2,951,865, Jafie et al., High Dielectric Constant Fluids and Plasticizers. Electroluminescent layer 8 may consist of any known electroluminescent phosphors such as zinc sulfide-zinc oxide with suitable activators such as copper, chlorine and manganese likewise dispersed in an organic polymeric matrix such as that used in connection with the insulating layer 7. A fibrous conducting sheet 9 is laid over the coated foil leaving a narrow margin thereof uncovered all around. Fibrous sheet 9 may consist of commercially available microfiber glass paper made conducting by dipping in a solution of indium basic trifiuoroacetate in an organic solvent and baking at an elevated temperature according to Patent 2,849,339, Iaffe, Indium Basic Trifiuoroacetate and Method of Coating Vitreous Bodies Therewith.

According to the preferred construction described in the aforementioned Fridrich and Dell patent, a thin sheet of low-density polyethylene was placed over the conducting glass paper 9 and conveniently was made of the same size as the outer encapsulating sheet 4, 5. During the heat-pressure laminating process for assembling the cell components, this low-density polyethylene sheet became substantially liquified and the glass paper sheet became partly embedded in it and also cemented by it to the phosphor layer 8 on the aluminum foil. We have now found that much improved results in respect of resistance to water depreciation may be achieved by using, instead of such low-density polyethylene sheet, a sheet of hydrophilic plastic material, that is a material having an afiinity for water or a disposition to retain water. The most suitable materials which we have found for this purpose are polyamide condensation polymers, for instance nylon 6 or nylon 6,6. Nylon 6 is the polyamide condensation product of caprolactam; nylon 6,6 is the polyamide condensation product of hexamethylenediamine and adipic acid. Other polyamides of a similar hydrophilic nature are likewise suitable. Nylon 6 is preferred and may be purchased in sheet form from B. V. Ludlow Papers Inc. of Needham, Massachusetts under the designation Caplene, and is available in thicknesses from .002" to .010". The thickness affects lamp life, as the thicker the film, the longer the time for which the onset of water depreciation is delayed. In making electroluminescent lamps for ordinary applications, I prefer to use a nylon film thickness of .005". The lamp constituents, and in particular the hydrophilic nylon layer, are dried out prior to encapsulation. Oven baking at 100 C. for one hour is used for nylon 6 of .005" thickness.

The electroluminescent cell is energized by applying an alternating potential between the conductive layers, that is between aluminum foil 6 and conductive glass paper 9. The projecting copper screens 2, 3 provide convenient terminals for so doing and they are connected respectively to copper ribbons 12, 13. The former, which may be relatively short, is laid under the aluminum foil 6 between the foil and the lower encapsulating plastic sheet 4. The latter is longer and extends over the length of the conducting glass paper 9, that is between the glass paper sheet 9 and the nylon sheet 10. During the laminating process, the copper ribbons become embedded in the thermoplastic sheets and are at the same time pressed against the aluminum foil or the conductive glass paper as the case may be. The ends of the copper ribbons and the copper screens 2, 3 likewise become embedded in the thermoplastic encapsulating sheets and are pressed in mutual contact and circuit continuity thereby achieved.

When the cell or lamp is lit, the phosphor is energized and lights up to the margin 9' of the conductive glass paper. The hand between the margin of the conductive glass paper at 9' and the coated aluminum foil from 9' to 6' is not illuminated and is opaque. The margin of the thermoplastic envelope projecting beyond the edge 6 of the aluminum foil does not light up but is transparent or at least translucent.

To laminate the various cell components together, the

eases stacked components or la '-up may be placed between press platens separated by a conformable diaphragm, suitably of aluminum foil. Compressed fluid is admitted over the diaphragm to exert hydrostatic pressure on the cell lay-up, vacuum is applied under the diaphragm to remove any trapped gases or moisture, and heat is supplied by suitable means to the lay-up in order to cause the plastic encapsulating sheets to soften and seal together at their margins. For more details of a suitable encapsulating technique, reference may be made to the aforementioned Fridrich and Dell patent.

It is believed that the water retentive plastic layer 10 operates somewhat as a desiccant or getter wherein water molecules diifusing through the encapsulating layer of polychlorotrifiuoroethylene are captured or retained. It has been proposed before now to provide a desiccant such as silica gel within a sealed electroluminescent lamp. These proposals have generally involved constructing the lamp as a hollow structure with an empty space left therein which is partially filled by a conventional desiccant such as silica gel. Evidently such an arrangement is not practical for a laminated electroluminescent lamp of the kind under consideration here utilizing a plastic encapsulating envelope which is evacuated and which has all its component layers pressed and adhering together. At-

tempts to make use of an ordinary desiccant according 7 to prior teachings have not until now resulted in a commercially acceptable structure.

Other plastic materials which exhibit the desired properties for a hydrophilic sheet are cellulose acetate and cellulose acetate butyrate. Films of polyvinyl alcohol, polyvinyl pyrrolidone and copolymers af maleic anhydride have also been used.

That the hydrophilic plastic layer of the instant lamp operates in a special manner appears to be confirmed by the manner in which life has been observed to vary with thickness of the layer. In FIG. 2, curves 15, 16 and 17 show the percentage of brightness as a function of hours of life in lamps according to the invention operated at relative humidity, for thicknesses of polycaprolactam or nylon 6 of .002, .004 and .006, respectively. Dotted line curve 18 shows typical dry depreciation, that is, under operation in 0% relative humidity. The ordinates are plotted on a reciprocal scale. It will be observed that the knee or break-off points A, B, C, representative of progressively greater thicknesses of the nylon 6 layer occurs at progressively greater time intervals from the origin. Prior to the occurrence of the knee, in each case the rate of depreciation is low and practically identical with the dry rate; likewise in each case beyond the knee, the rate of depreciation increases rapidly. It thus appears that the water retentive plastic layer is substantially completely effective in preventing water from passing through and deleteriously affecting the phosphor layer until such time as a given saturation has occurred; thereafter water passes through at a rate determined by the outer encapsulating plastic sheet of polytrifluoromonochloroethylene in conjunction with the external atmospheric conditions of relative humidity.

Nylon is preferred for yet another reason. It has been observed that with the use of a water retentive layer of nylon 6, an increase in brightness of approximately 20% occurs by comparison with otherwise identical electroluminescent cells utilizing the low density polyethylene sheet of the aforementioned Fridrich and Dell patent. The reason for the higher brightness with the nylon layer is not perfectly understood but it appears to be due to slight conductivity of the nylon supplementing that of the conductive glass paper layer. It is possible that the nylon layer serves as a conducting bridge between imperfectly conductively coated or fractured fibers of the glass fiber layer and thereby improves the uniformity or intensity of the electric field applied across the phosphor layer. Superior light transmittance and match of the refractive index to that of the fibrous conductive glass layer may also be factors.

The preferred embodiment of the invention which has been illustrated and described herein in detail is intended as illustrative and not in order to limit the invention thereto. The scope of the invention is to be determined by the appended claims.

What We claim as new and desire to secure by Letters Patent of the United States is:

l. A laminated evacuated electroluminescent lamp assembly comprising an outer envelope of plastic sheets of which at least the front sheet has a relatively high impermeability to water vapor and is light-transmitting, said outer platsic sheets being sealed together around their marginal edges and enclosing an electrically active assembly including from rear to front a conducting metal layer coated with an electroluminescent phosphor, a sheet of conductive light-transmitting material over said phosphor layer, and a sheet of hydrophilic plastic material over said conductive light-transmitting sheet and in contact with said front sheet of highly impermeable plastic whereby the combination of said front sheet of highly impermeable plastic backed up by said sheet of hydrophilic material serves to delay substantially the onset of water depreciation in said lamp.

2. A laminated evacuated electroluminescent lamp assembly comprising an outer envelope of light-transmitting thermoplastic sheets of relatively high water imperme ability, said thermoplastic sheets being sealed together around their marginal edges and enclosing and overreaching at their marginal edges an electrically active assembly including a conducting foil coated with an insulating layer of high dielectric constant material dispersed in a plastic matrix and over-coated with an electroluminescent phosphor layer dispersed in a plastic matrix, a sheet of conductive fibers over said phosphor layers, and a sheet of hydrophilic plastic material over said fiber sheet, said hydrophilic plastic material penetrating said fiber sheet and adhering to both said phosphor layer and the outer thermoplastic sheet.

3. A laminated evacuated electroluminescent lamp assembly comprising an outer envelope of plastic sheets of which at least the front sheet has a relatively high impermeability to Water vapor and is light-transmitting, said outer plastic sheets being sealed together around their marginal edges and enclosing and over-reaching at their marginal edges an electrically active assembly including from rear to front a conducting metal layer coated with an electroluminescent phosphor, a sheet of conductive light-transmitting material over said phosphor layer, and a sheet of nylon over said conductive lighttransmitting sheet and in contact with said front sheet of highly impermeable plastic whereby the combination of said front sheet of highly impermeable plastic backed up by said nylon sheet serves to delay substantially the onset of water depreciation in said lamp.

4. A flexible laminated evacuated electroluminescent lamp assembly comprising in stacked sequence a lowermost thermoplastic sheet of relatively highly water-impermeable material, a metal foil coated on its upper face with a layer of high-dielectric constant material embedded in a plastic matrix and overcoated wtih a layer of electroluminescent phosphor likewise embedded in a plastic matrix, said foil having its edges spaced from the edges of the lowermost sheeet, a sheet thereover of conductively coated fibers leaving an uncovered margin around the edges of the coated foil, a sheet thereover of nylon approximately coextensive with the sheet of conductively coated fibers, and a sheet thereover of thermoplastic relatively highly water-impermeable material forming an uppermost sheet coextensive with said lowermost sheet, said lowermost and uppermost sheets being fused together along their margins and said sheets of conductively coated fibers being embedded in and penetrated by the nylon sheet, said nlyon sheet adhering to the phos phor layer and to the uppermost thermoplastic sheet.

5. A flexible laminated evacuated electroluminescent lamp assembly comprising in stacked sequence a lowermost polychlorotrifiuoroethylene sheet, a soft annealed aluminum foil coated on its upper face with a layer of high-dielectric constant material embedded in a plastic matrix and overcoated with a layer of electroluminescent phosphor likewise embedded in a plastic matrix, said aluminum foil having its edges spaced from the edges of the lowermost sheet, a sheet thereover of conductively coated glass fibers leaving an uncovered margin around the edges of the coated aluminum foil, a sheet thereover of nylon 6 approximately coextensive with the aluminum foil, and thereover an uppermost sheet of polychlorotrifluoroethylene coextensive with said lowermost sheet, said lowermost and uppermost sheets being fused together along their margins and said fibrous glass sheet being embedded in and penetrated by the sheet of nylon 6, said sheet of nylon 6 adhering to the phosphor layer and to the uppermost polychlorotrifluoroethylene sheet.

References fired in the file of this patent UNITED STATES PATENTS 2,901,652 Fridrich Aug. 25, 1959 2,944,177 Piper July 5, 1960 2,945,976 Fridrich et a1. July 19, 1960 

1. A LAMINATED EVACUATED ELECTROLUMINESCENT LAMP ASSEMBLY COMPRISING AN OUTER ENVELOPE OF PLASTIC SHEETS OF WHICH AT LEAST THE FRONT WHEET HAS A RELATIVELY HIGH IMPERMEABILITY TO WATER VAPOR AND IS LIGHT-TRANSMITTING, SAID OUTER PLASTIC SHEETS BEING SEALED TOGETHER AROUND THEIR MARGINAL EDGED AND ENCLOSING AN ELECTRICALLY ACTIVE ASSEMBLY INCLUDING FROM REAR TO FRONT A CONDUCTING METAL LAYER COATED WITH AN ELECTROLUMINESCENT PHOSPHOR, A SHEET OF CONDUCTIVE LIGHT-TRANSMITTING MATERIAL OVER SAID PHOSPHOR LAYER, AND A SHEET OF HYDROPHILIC PLASTIC MATERIAL OVER 